Processes for making paper pulp consist in reducing the raw materials to separate fibers containing a greater or lesser amount of cellulose depending on the qualities which the pulp produced is required to have. The processes essentially consist of grinding operations, which are basically mechanical, which may be combined with more or less powerful delignification operations, which are basically chemical.
Depending on the relative importance of the two treatments, it is possible to distinguish five major types of pulp:
(1) Mechanical pulp, obtained by grinding without any chemical treatment beforehand of the raw material;
(2) Thermo-mechanical pulp, obtained by grinding under pressure, which is made easier by steaming the raw material beforehand to soften the lignin;
(3) Mechano-chemical pulp, obtained by grinding in combination with in situ or ex situ preliminary treatment of the raw material with chemical reagents;
(4) Semi-chemical pulp, obtained by grinding raw material which is previously subjected to partial chemical “cooking” under pressure; and
(5) Chemical pulp, where the chemical processing is much more powerful and produces both the delignification and the major part of the reduction to fiber.
Refiner mechanical pulp (RMP) is produced by the mechanical reduction of wood chips (and sometimes sawdust) in a disc refiner. The process usually involves the use of two refining stages operating in series, i.e., two-stage refining, and produces a longer-fibered pulp than conventional ground wood. As a result, it is stronger, freer, bulkier, but usually somewhat darker in color, than stone ground wood. Thermomechanical pulping (TMP) was the first major modification of RMP, and is still employed on a large scale to produce high-tear pulps for newsprint and board. This process involves steaming the raw material under pressure for a short period of time prior to and during refining. The steaming serves to soften the chips, with the result that the pulp produced has a greater percentage of long fibers and fewer shives than RMP.
It is becoming increasingly important to produce TMP pulp that is both uniform and of a high quality. Papermakers desire to optimize paper machine operations, and in some instances to replace the expensive kraft furnish. Even though advanced process control has gained general acceptance in the pulp and paper industry, the thermo-mechanical pulping process is still under manual control in most pulp mills. Reliance on manual control stems primarily from the complexity of the TMP process, which is highly interactive requiring control and variable inputs from many sections of the refining process. Additionally, control of the TMP process is further complicated as blow-line consistency in most cases is not measured using an online sensor. Pulp quality descriptive variables such as fiber length and freeness are also measured infrequently.
In order to produce high quality thermo-mechanical pulp, the refining process must be under tight control. Closed loop control of a TMP refiner system is one of the most complex and challenging control problems in a pulp mill. The process is inherently multivariable, exhibiting strong interactions. In addition, the bandwidth of the process outputs is spread over a wide frequency range. For example, the open loop response between the primary plate gap to the primary motor load and final pulp freeness is about 2 minutes and about 90 minutes, respectively. The refining process is also complicated by non-stationary process dynamics due to wear of the refiner plates.
In the past, TMP controller design was attempted using single-loop PID based decentralized control architecture. The choice of decentralized architecture was appealing as it was easy to understand by mill personnel and simple to implement using the existing distributed control system (DCS). Proportional-integral-derivative (PID) based control strategy is acceptable for regulation of local control loops such as flow and pressure regulation, but a PID controller cannot handle complex multivariable dynamics. In addition, a PID controller can only control a single process output. However, to adequately control pulp quality at least two variables such as, for example, Canadian standard freeness (CSF) and mean-fiber length (MFL) must be controlled. Since these variables are physically linked, they cannot be independently controlled to arbitrary targets, instead these variables must be controlled within an operator defined quality window. The quality window is defined by setting the upper and lower limits on the pulp quality variables. In order to handle this control problem a multivariable controller is required that can also handle process constraints. Constrained model based predictive control (MPC) is a natural candidate in the process industrial. MPC provides a unified framework to efficiently handle complex process interactions and constraints. MPC technology has also gained industrial acceptance and it can be easily integrated into existing mill DCS platforms.
The use of MPC to control a refiner system has been presented by Du, H., entitled, Multivariable predictive control of a TMP plant, Ph.D. dissertation, UBC, Vancouver, BC, Canada, 1998. The study was presented at a theoretical level that demonstrated the need to control the refining intensity. However, no attempt was made to directly control the pulp quality. Other work by Strand, W. C., et al., IMPC 2001 discusses the use of MPC but the details of the control strategy are not disclosed.